Japanese Patent Application Publication No. 2010-114015 discloses a fluorescent display having an electrostatic capacitance type touch switch mounted on a front plate of an envelope of the fluorescent display. Referring to FIGS. 4 and 5, this fluorescent display 10 includes an envelope 14 constituted of a substrate 11, side plates 12 and a front plate 13 made of glass. As shown in FIG. 5, the fluorescent display 10 includes an anode 16 having a phosphor layer 15 arranged on the substrate 11 inside the envelope 14, a control electrode 17 arranged above the anode 16, and filament-shaped cathodes 18 as an electron source arranged above the control electrode 17. Electrons emitted by the cathode 18 are controlled by the control electrode 17 and impinge on the anode 16, producing the luminescence of the phosphor layer 15. The luminescence of the phosphor layer 15 is visually observed through the front plate 13.
As shown in FIGS. 4 and 5, a frame-shaped, insulating light-shielding layer 19 covers a predetermined portion of an inner face of the front plate 13 adjacent to an outer periphery of the front plate 13, except for terminal portions. A portion surrounded by the light-shielding layer 19 corresponds to a display window 20. In addition, switch electrodes 21, 22 are provided on the inner face of the front plate 13 corresponding to the display window 20.
As shown in FIG. 4, the switch electrodes are arranged on the display window 20, the switch electrodes including the switch electrodes 21 and the linear switch electrodes 22 arranged in sets of four (22a-22d) and capable of outputting with position resolution of for example 128. Furthermore, a shield electrode 30 is provided between the cathode and some of the switch electrodes 21, 22 to prevent the switch electrodes 21, 22 from being charged with electrons.
Referring now to FIG. 6, there is shown a positional relationship of the switch electrodes 21 and the patterned phosphor layers 15 arranged within the outline of the substrate 11. FIG. 6 shows exemplary patterns of the phosphor layers 15, including a pattern 15a indicating “HDD”, a pattern 15b indicating “USB”, a pattern 15c indicating a logo featuring a mail and a pattern 15d indicating a bar. In addition, the dotted rectangles overlapped on the respective patterns indicate positions of the switch electrodes 21 and the linear switch electrodes 22a-22d. 
When a finger of a user touches on an outer face of the front plate 13 having the switch electrodes 21 related to the respective phosphor layers 15, the electrostatic capacitance of this switch electrode 21 changes. A switch control circuit detects this change in capacitance, judges whether it corresponds to ON or OFF of the switch and provides outputs as ON or OFF of the switch. Then, this output is received by a display control part which then controls the phosphor layer 15 corresponding to the switch to light up or not to light up based on the ON or OFF of the switch.
When the finger of the user touches on the outer face of the front plate 13 corresponding to the linear switch electrode 22, the output value from the linear switch electrode 22 continuously changes according to the position on the front plate 13. Thus, the light up of the bar can be controlled.
Referring now to FIG. 7, the following describes the operation of the above-described touch switch. A control part 23 includes a pulse generator 24, a comparator circuit 25 and a capacitor C connected between the pulse generator 24 and one input of the comparator circuit 25. The switch 21 is connected between the pulse generator 24 and the other input of the comparator circuit 25.
When the finger of the user touches on the outer face (a touch portion S) of the front plate 13 corresponding to the switch electrode 21, the electrostatic capacitance between the finger and the switch electrode 21 is induced, producing a kind of a capacitor. In addition, the capacitor C connected between the pulse generator 24 and one input of the comparator circuit 25 is arranged to have the electrostatic capacitance substantially equal to that of the switch electrode 21 not in contact with the finger.
When the finger of the user touches the touch portion S, then the outer face contacting the finger serves as a dielectric material, thereby changing the electrostatic capacitance of the switch electrode 21. Thus, the produced difference in the electrostatic capacitance between the switch electrode 21 and the capacitor C causes the difference in pulse voltages applied to the both inputs of the comparator circuit 25, producing an output from the comparator circuit 25.
FIG. 8 shows an example of the above-described conventional fluorescence display 10. As shown, there is provided a dummy pattern 28 which covers the display window 20, except for the regions corresponding to the switch electrodes 21, the linear switch electrodes 22a-22d and leading wires 27 connecting the switch electrodes 21, 22a-22d and terminal portions 26.
The dummy pattern 28 is formed by strip or mesh-shaped thin wires arranged at a predetermined interval. These thin wires are made of a metallic thin film having the width equivalent to that of the switch electrode 21. This dummy pattern 28 functions to equalize the transmittance throughout the display area. This dummy pattern 28 also prevents an error in reaction of the touch switch.
FIG. 9 shows an enlarged photograph of the fluorescent display 10 having the conventional electrostatic capacitance type touch switch explained above. FIG. 9 shows an example of an actual structure of the touch switch and the dummy pattern. The switch electrode 21 as a touch switch is arranged within a frame at an outer periphery and is formed by slanted strip-shaped thin wires arranged at a predetermined interval with respect to each other. Also, the dummy pattern 28 located next to the switch electrode 21 is formed by electrodes arranged similar to the switch electrode 21. Furthermore, the leading wire 27 connecting the switch electrode 21 with an eternal component extends linearly from the frame. The leading wire 27 is arranged at a predetermined interval from the frame of the dummy patterns 28 located next to the switch electrode 21. Thus, a gap between the switch electrode 21 and the dummy pattern 28 is formed into a band-like shape having a predetermined width defined by the frame of the switch electrode 21 and the dummy pattern 28. Similarly, a gap between the leading wire 27 of the switch electrode 21 and the frame of the dummy pattern 28 is formed into a band-like shape having a predetermined width.
Referring now to FIG. 3, the graph shows the electrostatic capacitance in a touch state in which a finger of a user is in contact with the touch switch and in a non-touch state in which a finger of a user is not in contact with the touch switch of the electrostatic capacitance type touch switch. The left graph (a) corresponds to the present invention and the right graph (b) corresponds to prior art as a comparative example. The control part 23 of the touch switch continuously measures the electrostatic capacitance, namely, count value, for the switch electrode 21 and determines whether the state is in the touch state or in the non-touch state based on the change in the count value. Generally, it is preferable that the count value in the non-touch state (i.e. a base capacitance) is small while the change in the count value in the touch state (i.e. a capacitance change) is large.
Thus, the control part 23 is arranged to change the base capacitance and the capacitance change by adjusting the sensitivity within the count value range below the upper limit of the count value determined by software installed in the control part 23. That is, the smaller the base capacitance is, the more the capacitance change can be increased and widened by increasing the counting scale within the sensitivity adjustable range. Thus, the sensitivity can be improved.
In the exemplary structure shown in FIG. 9, the width of the leading wire 27 is 0.05 mm, the gap between the switch electrode 21 and the dummy pattern 28 is 0.06 mm, the width of the thin wire for both of the switch electrode 21 and the dummy pattern 28 is 0.03 mm, the interval between the respective thin wires for both of the switch electrode 21 and the dummy pattern 28 is 0.18 mm, and the transmittance of both of the switch electrode 21 and the dummy pattern 28 is 83%
However, there is a problem in the conventional electrostatic capacitance type touch switch having the structure shown in FIG. 9. That is, a boundary between the switch electrode 21 and the dummy pattern 28 located next to each other at the predetermined interval, for example 0.06 mm in the touch switch shown in FIG. 9 is visually prominent and appealing. Similarly, a boundary between the dummy pattern 28 and the leading wire 27 of the switch electrode 21 located next to each other at the predetermined interval, for example 0.06 mm in the touch switch shown in FIG. 9, is also visually prominent and appealing. Thus, the appearance of the touch switch is not good.
Furthermore, there is another problem in the conventional electrostatic capacitance type touch switch shown in FIG. 9. That is, the gap between the switch electrode 21 and the dummy pattern 28 as well as the gap between the leading wire 27 of the switch electrode 21 and the frame of the dummy pattern 28 are formed into a band-like shape. Thus, the capacitance between the switch electrode 21 and the dummy pattern 28 or the capacitance between the leading wire 27 and the frame of the dummy pattern 28 becomes great with an increase in the size of the structure. Thus, as shown in the right graph (b) of FIG. 3, the base capacitance for the non-touch state is very close to the upper limit of the count value determined by the performance of the software of the control part 23. Thus, if the difference in the capacitance in the touch state and the non-touch state is the same for the case with the base capacitance being very close to the upper limit of the count value as described above and the case with the relatively small base capacitance, the difference in the capacitance with respect to the base capacitance becomes relatively small for the case with the base capacitance being very close to the upper limit of the count value compared to the case with the relatively small base capacitance. Thus, it is difficult to improve the accuracy of detecting a touch based on the change in capacitance by enhancing the sensitivity by increasing the difference in capacitance within the count range by adjusting the sensitivity, as described above.